1) Field of the Invention
The present invention relates to an image forming apparatus that forms an image on an image carrier, and records the image on a recording medium, an optical scanner that scans the image carrier to write the image on the image carrier, and a scanning lens that transmits a light for scanning.
2) Description of the Related Art
Color image forming apparatuses equipped with optical scanners that irradiate optical beams onto surfaces of a plurality of image carriers to be scanned and write a latent image on the image carriers include full-color copiers, optical printers, facsimiles, and plotters.
In the optical scanners mounted on the image forming apparatuses, it is desired that positional misalignment (relative location-related characteristics) of scanning lines is not formed on a surface of the image carrier to be scanned. Examples of the misalignments of scanning lines are illustrated in FIG. 21 to FIG. 26.
The line of an image formed on a transfer paper P extending in a main scanning direction is indicated by broken line, and the line of an ideal position is indicated by solid line respectively. The main scanning direction refers to a direction in which the optical scanner writes in the recording medium, and a sub-scanning direction refers to a direction in which the recording medium moves orthogonal to the main scanning direction. Moreover, the solid line and the broken line shown in FIG. 21 to FIG. 26 have an overlapping part with each other. However, for convenience of drawing, the broken line is illustrated at a position slightly shifted from the solid line in the sub-scanning direction.
FIG. 21 is a schematic diagram for illustrating a case where scanning lines extending in a main scanning direction is deviated in parallel along the sub-scanning direction (resist deviation). Poor performance of an optical element such as a lens disposed on an optical path of the optical scanner, inaccuracy of geometric arrangement of the respective optical elements, and a displacement due to thermal expansion of the respective optical elements and holding members cause such a problem.
FIG. 22 is a schematic diagram for illustrating a case where the scanning line is inclined to the sub-scanning direction with respect to an ideal scanning line extending in the main scanning direction. Poor performance of the optical elements and inaccuracy of the geometric arrangement of the respective optical elements cause this problem.
FIG. 23 is a schematic diagram for illustrating a case where the scanning line is curved in the sub-scanning direction with respect to the ideal scanning line extending in the main scanning direction. Poor performance of the optical elements and inaccuracy of the geometric shape or deformation of the respective optical elements cause this problem.
FIG. 24 is a schematic diagram for illustrating a case where a resist deviation in the main scanning direction is generated in a writing position of the scanning line with respect to the ideal scanning line extending in the main scanning direction. This is caused by difference in tilt of a plane of each mirror of a plurality of mirrors provided in a polygon mirror reflector, or by difference in an amount of light according to an image forming mode. Moreover, when a multi-beam scanning method is used, which is a method for forming N scanning lines in the sub-scanning direction using a plurality of laser diodes (LDs) by single optical scanning, the above resist deviation is generated due to a slight difference in each LD wavelength.
FIG. 25 is a schematic diagram for illustrating a case where magnification deviation is generated in the scanning line with respect to the ideal scanning line extending in the main scanning direction. This is caused by poor performance of the optical element and inaccuracy of the geometric arrangement of the respective optical elements. Further, this is caused by a displacement due to the thermal expansion of the respective optical elements and the holding members, or a slight difference in each LD wavelength when the multi-beam scanning method is used.
FIG. 26 is a schematic diagram for illustrating a case where a position of the scanning line written actually dose not correspond to an ideal position because scanning speed in the main scanning direction is microscopically different. This is caused by poor performance of the optical element and inaccuracy of the geometric arrangement of the respective optical elements, and a displacement due to the thermal expansion of the respective optical elements and the holding members.
Conventionally, as described in Japanese Patent Laid Open No. 11-72732, for example, some of the optical scanners have an arrangement to prevent deterioration of fθ characteristics generated due to an arrangement error, which is generated from a processing error of a lens and a processing error of an optical housing for supporting the lens, by rotatably adjusting the lens.
The fθ characteristics can surely be prevented. However, an adjusting shaft cannot be decided uniquely, and positions in an optical axis direction and in the main scanning direction are changed simultaneously with rotation of the lens, resulting in another problem that other characteristics are deteriorated.
In addition, as shown in FIG. 27A, in some cases, by rotatably adjusting the lens, the inclination of the scanning line shown in FIG. 22 is corrected.
Reference numeral 11 in the figure indicates an optical housing. An engagement projection 12 is provided in the center of the bottom of the optical housing 11 in an upper direction. An engaging groove 12a is provided on a tip surface of the engagement projection 12, and a projecting part 13a is fitted into the engagement groove 12a, thereby disposing a long-sized toroidal lens 13 within the optical housing 11.
One end of the long-sized toroidal lens 13 is placed on a fixing projection 14 provided in the optical housing 11. The other end thereof is placed on a feed screw 15. The feed screw 15 is fitted to a tip of a driving shaft of a drive motor 16, and screwed to the optical housing 11. The drive motor 16 is fixed to the optical housing 11 with a fixing screw 17. The toroidal lens 13 within the optical housing 11 is pressed from upside by a plurality of leaf springs 18. The leaf springs 18 are supported respectively by a bracket 19 to be fixed by screwing to the optical housing 11.
The toroidal lens 13 is formed by setting an optical direction as the X-axis, a corresponding main scanning direction orthogonal thereto as the Y-axis, and a corresponding sub-scanning direction orthogonal to those as the Z-axis. The X, Y, Z axially rotating directions are set to α, β, and γ, respectively.
Then, a position in the X-axial direction is decided by hitting projection pieces 13b and 13c of both ends of the toroidal lens 13 to a part (not shown) of the optical housing 11 respectively by energizing with the leaf springs (not shown). A position in the Y-axial direction is decided by fitting the projection part 13a into an engagement groove 12a. A position of the Z-axial direction is decided by pressing both ends of the toroidal lens 13 against the fixing projection 14 and the feed screw 15 with the leaf springs 18.
When correcting the inclination of the scanning line, the feed screw 15 is screwed in by driving the drive motor 16, thereby rotating the toroidal lens 13 in a direction of α by setting the fixing projection 14 as a supporting point. Then, as shown in FIG. 27B, an optical axis L is moved from an ideal position, and the line N connecting vertexes R of the toroidal lens 13 is positioned extremely different from the passing position M of the ideal scanning line. Accordingly, the image pick-up performance in a direction of Z, that is, in a corresponding sub-scanning direction is deteriorated. In a lens having power in the corresponding sub-scanning direction such as the toroidal lens 13, it is conventionally known that a scanning line curve as shown in FIG. 23 can be changed by changing the curve in the direction of Z so that the inclination of the scanning line can be adjusted by rotational adjustment.
The Japanese Patent Laid Open No. 11-72732 discloses a method for adjusting the scanning line curve as shown in FIG. 23, by fixing both end parts of a cylindrical lens corresponding to the aforementioned toroidal lens 13 in the direction of Z and moving a center part thereof.
However, even in this example, it is easily conceivable that the optical axis of the lens is undesirably moved from a target position by adjusting the scanning line curve. This also results in deteriorating the pick-up performance in the sub-scanning direction.
Moreover, for example, the conventional optical scanner includes the one described in Japanese Patent Laid Open No. 2001-142012 as an optical scanner capable of changing the position of the scanning line in the sub-scanning direction.
The Japanese Patent Laid Open No. 2001-142012 discloses the optical scanner in which a part formed in a hemispherical shape provided at tip of a rod is pressed against the longitudinal center lower part of the mirror formed on an optical path by energizing force of a spring, back end side of the rod is engaged with gear parts of a stepping motor through a different gear, the stepping motor is rotated, thereby causing the rod to perform forward/backward motion, and according to a moving amount of the rod, an angle of reflection of the mirror in the sub-scanning direction is changed.
However, in a method conducted by the aforementioned conventional optical scanner for adjusting a resist deviation of the scanning line in the sub-scanning direction by adjusting emission timing, a minimum adjusting resolution corresponds to one-scan in the main scanning direction. For example, when the adjusting resolution is 600 dpi, a scanning range becomes larger to be about 42.3 μm, thereby lowering the alignment accuracy.
In addition, the Japanese Patent Laid Open No. 2001-142012 discloses an optical scanner in which with the rotation of the stepping motor, a plurality of gears are accordingly rotated to cause the rod to conduct forward/backward motion. Then, by moving back and forth of the rod, the position in the central lower end part of the mirror is changed, thereby changing the angles of reflection of the mirror in the sub-scanning direction. Therefore, an amount of change in the sub-scanning direction of the mirror with respect to the angles of displacement of the mirror becomes large, thereby posing a problem such that resolution becomes large accordingly.